Oxygen sensors generally have short lifetimes such as less than 100 hours or have a high sensitivity to temperature variations (such as 0.01 PPO2/degree C. for fluorescent based sensors). For example, rebreathers are currently are limited to fuel cell based oxygen sensors and because these are chemical-based sensors, they require frequent calibrations, and have a limited lifetime thus making these sensors generally unreliable. Maximal oxygen consumption (VO2) measurements are generally limited to using CO2 sensors to infer oxygen levels, which can require significantly greater amounts of space and power than some types of oxygen sensors.